Electron source

ABSTRACT

The invention concerns a source supplying an adjustable energy electron beam, comprising a plasma chamber (P) consisting of an enclosure ( 1 ) having an inner surface of a first value (S 1 ) and an extraction gate ( 2 ) having a surface of a second value (S 2 ), the gate potential being different from that of the enclosure and adjustable. The invention is characterized in that the plasma is excited and confined in multipolar or multidipolar magnetic structures, the ratio of the second value (S 2 ) over the first value (S 1 ) being close to: D=1/β {square root}2πcm e /m i  exp (−½), wherein: β is the proportion of electrons of the plasma P, m e  the electron mass, and m i  is the mass of positively charged ions.

The present invention relates to the field of electron sources. Morespecifically, the present invention relates to the forming of extendedelectron beams intended to be injected into a plasma, under vacuum, orin any reactive gaseous atmosphere at reduced pressure.

On manufacturing of circuits in semiconductor devices, certaindeposition or etch steps performed in ionized gases use electronsources. Indeed, it may for example appear to be necessary to negativelybias the surface of a substrate by bombarding it with an electron beam.An electron source may further be necessary to generate a plasma, toincrease the ionization rate of a plasma, or yet to accelerate the ionsof a plasma in which a substrate is placed. Further, different electronbeam powers are desired to be used according to the consideredapplication. For example, it may be necessary to have an electron beamwith a power on the order of 100 eV to enable erosion (etch) of asubstrate.

Electron sources of point type such as a heated emissive cathode arealready available. However, such sources have the disadvantage ofproviding beams with a very small cross-section. The maximum possibleprocessing surface area is thus very limited. Further, such sources canseldom be used in the considered applications, since the involved gases(plasma) risk reacting with the electrode.

Part 5.5.2 “Electron beam characteristics of miniaturized ECR plasmacathodes” of work “Electron Cyclotron Resonance Ion Sources and ECRPlasmas” by R. Geller, published by the Institute of Physics PublishingBristol and Philadelphia (pages 352-353) describes electron sources inwhich electrons are extracted from a plasma. However, such sourcescannot be sources of large surface area. Indeed, to be able to increasethe electron extraction surface area of a plasma, the plasma volume hasto be increased. Then, obtaining the electromagnetic field necessary toexcite the plasma at the cyclotron resonance imposes generation of acontinuous axial magnetic field of excessively high intensity. Thegeneration of such a magnetic field, intended to confine the trajectoryof the extracted electrons around the magnetic field lines imposes usingparticularly complex, bulky, and expensive installations. Further, thepresence of a continuous axial magnetic field may be a problem incertain applications. This limits the extraction surface area to a fewsquare millimeters. Further, electron sources of this type generateelectron beams of an intensity limited to the order of one hundredmilliamperes. Further, the current density exhibits an exponential-typevariation along with the beam extraction power. It is thus impossible toextract on an extended surface area an electron beam of a currentdensity which remains relatively steady when the beam extractionpotential is modified. Further, said potential is at most on the orderof some hundred volts.

The present invention aims at providing an electron source whichexhibits at least some of the following features:

-   -   usable for different application fields,    -   capable of providing a beam, with as high as possible an        electron current density,    -   relatively steady when the beam power is varied,    -   beam power controllable within a wide range,    -   surface area as extended as possible, for example, with a        diameter from a few centimeters to a few tens of centimeters, or        more,    -   free of the problems of known sources, and especially,        exhibiting no axial magnetic field at the level or downstream of        the extraction.

To achieve these objects, the present invention provides a sourceproviding an electron beam of settable power, comprising a plasmachamber formed of an enclosure having an internal surface area of afirst value and of an extraction grid having a surface area of a secondvalue, the grid potential being different from that of the enclosure andbeing settable, characterized in that the plasma is excited and confinedin multipolar or multidipolar magnetic structures, the ratio of thesecond value to the first value being close to the following upperlimit:${D = {\frac{1}{\beta}\sqrt{\frac{2\quad\pi\quad m_{e}}{m_{i}}}{\exp\left( {{- 1}/2} \right)}}},$where:

-   -   β is the electron ratio of plasma P,    -   m_(e) is the mass of the electron, and    -   m_(i) is the mass of the positively-charged ions,        whereby the electron current density of the extracted beam is        substantially steady when the grid-enclosure potential        difference varies.

According to an embodiment of the present invention, the plasma isexcited and confined by microwaves at the distributed electroniccyclotron resonance.

According to an embodiment of the present invention, the ratio betweenthe second value and the first value is selected to be between D/10 andD.

According to an embodiment of the present invention, the ratio betweenthe second value and the first value is selected to be between D/2 andD.

According to an embodiment of the present invention, the grid is dividedinto a plurality of elementary grid portions.

According to an embodiment of the present invention, various gridportions are gathered and set to distinct potentials.

According to an embodiment of the present invention, the source isfollowed by a post-acceleration enclosure.

According to an embodiment of the present invention, the gate is formedof parallel tubes run through by a cooling fluid.

The foregoing objects, features, and advantages of the present inventionwill be discussed in detail in the following non-limiting description ofspecific embodiments in connection with the accompanying drawings, amongwhich:

FIG. 1 schematically illustrates a charged particle extraction plasmachamber;

FIG. 2 illustrates the electronic and ionic currents at the level of asurface of a chamber according to FIG. 1; and

FIG. 3 schematically illustrates an alternative electron sourceaccording to the present invention.

According to the present invention, a plasma chamber using excitationand confinement structures using multipolar or multidipolar magneticstructures at the distributed electron cyclotron resonance such asdescribed, for example, in French patent applications No 85/08836,93/02414, 94/13499, and 99/10291, is used as an electron source.

Such low-pressure plasma excitation devices advantageously enableincreasing the surface area of extraction of an electron beam withoutimposing large magnetic field volumes, and while avoiding the presenceof a magnetic field at the level and downstream of the extraction.

FIG. 1 schematically and partially illustrates a specific embodiment ofthe present invention. A confinement chamber of a plasma P is formed ofan enclosure 1 with an internal surface area S₁ and of an extractiongrid 2 of surface area S₂. Grid 2, isolated from enclosure 1, is biasedby a voltage VB to potential V₂ with respect to this enclosure. VoltageVB is settable by a used. Electrons are desired to be extracted fromplasma P. Potential V₂ of grid 2 must be greater than potential V₁ ofenclosure 1, that is, voltage VB is positive (V_(B)=(V₂−V₁)>0). Themultipolar or multidipolar magnetic excitation structures are not shownin FIG. 1. These will be, for example, structures similar to thosedescribed in the above-mentioned French patent applications.

The electron beam thus generated penetrates through grid 2 into anenclosure 3 of confinement of a processing atmosphere (gas, ionized gas,or plasma) of an element to be processed 4, such as a semiconductorsubstrate. The beam coming from grid 2 penetrates into enclosure 3 andenables processing and/or negatively biasing a surface S₄ of substrate4.

FIG. 2 illustrates the theoretical variation of the currentscorresponding to the particles extracted from plasma P according to thevalue of extraction voltage V_(B). An electron current I_(e) isconsidered as being positive and a positive ion current I_(i) isconsidered as being negative. More specifically, it is known, forexample from French patent application No 95/04729 issued to the CNRS,that the electron current I_(e) and the positive ion current I_(i) thatcan be extracted from plasma P depend on difference U=V_(B)−V_(P), whereV_(P) is the potential of plasma P. When U is negative and rangesbetween V_(f) and zero, electron current I_(e) increases according to anexponential law to a maximum saturation value I_(esat). When U ispositive, that is, when bias potential V_(B) exceeds the potential ofplasma V_(P), the extracted electron current remains constant at maximumsaturation voltage I_(esat). However, the extracted positive ion currentI_(i) remains substantially constant as long as U is negative. When U ispositive, ion current I_(i) becomes zero. FIG. 2 also illustrates indotted lines real current I_(r), that is, the algebraic sum of electronand positive ion currents I_(e) and I_(i). The value of U for whichelectron current I_(e) is equal to positive ion current I_(i), that is,for which real current I_(r) is zero, is called the floating potentialV_(f).

A priori, to obtain a source such as defined hereabove, it should beenough to apply a voltage V_(B) sufficiently high to be in thesaturation field, to the right of the curve of FIG. 2. In practice,account must be taken of an adjustment with respect to one another ofpotential V_(P) of plasma P and of potentials V₁ of enclosure 1 and V₂of grid 2. Indeed, such a self-adjustment enables guaranteeing at anytime the neutrality of plasma P. To keep the neutrality principle, thepositive ion current on all electrode surfaces must at any time exactlycompensate for the electron current on these surfaces.

To reach saturation I_(esat) on grid 2, the characteristic of FIG. 2shows that the adjustment must be performed so that potential V₂ of grid2 is greater than potential V_(P). Similarly, to respect the neutralityprinciple, that is, so that surface area S₁ of enclosure 1 can absorbthe ion current compensating for this saturation current I_(esat),potential V₁ of enclosure 1 must be smaller than potential V_(f).Surface area S₁ then receives both an ionic saturation current and anelectronic current, as illustrated in FIG. 2.

In steady state, the saturation electronic current towards grid 2 isgiven by the following relation:${I_{2} = {I_{esat} = {{- e}\quad\beta\quad{nS}_{2}\sqrt{\frac{k_{B}T_{e}}{2\quad\pi\quad m_{e}}}}}},$where:

-   -   −e is the electronic charge,    -   β is the electron ratio of plasma P,    -   n the ion density of plasma P,    -   S₂ the surface area of grid 2,    -   k_(B) is Boltzmann's constant,    -   T_(e) is the electron temperature in the beam, and    -   m_(e) is the mass of the electron.

The current collected by enclosure 1 is the sum of a positive ionicsaturation current and of an electronic current and can be expressed asfollows:${I_{1} = {{{enS}_{1}\sqrt{\frac{k_{B}T_{e}}{m_{i}}}{\exp\left( {{- 1}/2} \right)}} - {e\quad\beta\quad{nS}_{1}\sqrt{\frac{k_{B}T_{e}}{2\quad\pi\quad m_{e}}}{\exp\left\lbrack \frac{e\left( {V_{1} - V_{P}} \right)}{k_{B}T_{e}} \right\rbrack}}}},$where m_(i) is the mass of the positively-charged ions.

Then, to respect to plasma neutrality condition, there must be I₁+I₂=0,which results in the following relation:${V_{P} - V_{1}} = {{- \frac{k_{B}T_{e}}{e}}{{\ln\left\lbrack {{\frac{1}{\beta}\sqrt{\frac{2\quad\pi\quad m_{e}}{m_{i}}}{\exp\left( {{- 1}/2} \right)}} - \frac{S_{2}}{S_{1}}} \right\rbrack}.}}$

Further, the automatic adjustment of the potentials must be performed sothat this difference is positive. The surface area ratio must thus besuch that:$\frac{S_{2}}{S_{1}} < {\frac{1}{\beta}\sqrt{\frac{2\quad\pi\quad m_{e}}{m_{i}}}{{\exp\left( {{- 1}/2} \right)}.}}$

To simplify the rest of the discussion, the optimal limit thus definedwill be called D, that is:$D = {{\frac{1}{\beta}\sqrt{\frac{2\quad\pi\quad m_{e}}{m_{i}}}{\exp\left( {{- 1}/2} \right)}} \cong {\frac{1.5}{\beta}\sqrt{\frac{m_{e}}{m_{i}}}}}$

This condition being fulfilled, surface area S₁ takes a potential V₁close to floating potential V_(f), potential V₂ of surface area S₂ takesa positive value with respect to plasma potential V_(p) and receives anelectronic current density equal to the electronic saturation currentdensity. The power of the electrons in the beam output by grid 2 then ison the order of extraction power eV_(B), assuming that the thermal powerof the electrons k_(B)T_(e) is negligible as compared to V_(B). The beampower thus only depends on the potential difference V_(B) applied by theuser between grid 2 and enclosure 1.

If the upper limit thus defined is exceeded, the obtained sourceexhibits an extended surface area, but the density of the electroniccurrent of the extracted beam is no longer substantially steady withrespect to the extraction power and varies exponentially with theextraction power.

According to the present invention, it is however possible to obtain abeam with an extended cross-section of substantially constant intensity(I_(esat)) and of controlled variable power (eV_(B)). More specifically,it is possible to vary at the output of the extraction chamber theelectron power within a range from a few eV to a few hundreds of eV,instead of some ten eV with prior devices. This result is obtainedwithout modifying the level of the extracted electronic current. Saidcurrent depends on the plasma density and may reach current densities offrom a few tens to a few hundreds of mA/cm².

According to the present invention, the current is substantially steady,that is, its variation according to extraction voltage U does not havean exponential character (of type e^(U)), but is of type U^(α), withα<½, for example according to a {square root}{square root over (U)} law.

According to the present invention, it is also possible to optimizeextraction surface area S₂. Surface area S₂ is chosen so that ratioS₂/S₁ of the extraction surface area to the internal surface area ofenclosure 1 of the chamber is smaller than limit D, but as close aspossible thereto.

To increase the surface area S₄ that can be processed, the grid may besplit as illustrated in FIG. 3, the sum of surface areas S₂₁, S₂₂, . . ., S_(2n) of the grid portions corresponding to above-mentioned surfacearea S₂. Indeed, by a beam dispersion effect, the processed surface areais greater than the extraction surface area. The openings are formedclose to one another so that the beams partially cover one another atthe level of element 4 as they scatter, so that surface area S₄ iscontinuously bombarded. All the grid portions can be biased to a samevoltage V_(B). It may also be chosen to gather the grid portions intoassemblies biased to distinct voltages.

Another advantage of this splitting of extraction surface area S₂ is toguarantee a better cooling down thereof. Indeed, if an electron currentwith a relatively high density I_(esat) is desired to be extracted, arelatively high thermal power generation can be observed at the gridlevel. Now, it is easier to cool down a plurality of elementary gridsthan a continuous extraction grid, especially because the surfaceseparating two elementary grids can be used as a radiator or cooled downby the flowing of a fluid. It is thus possible according to the presentinvention to ensure an efficient cooling down even for relatively highextraction powers while guaranteeing an extended extraction surfacearea. For the extraction from plasmas of very high densities, a directcooling down of the grid will have to be provided, by forming it bymeans of a bundle of parallel tubes of a diameter on the order of one mmspaced apart by a distance on the order of one mm.

As a non-limiting example, if the plasma is an argon plasma havingatomic number 40, β=1 and ratio S₂/S₁ must be smaller thanD=1.5/(1836.40)^(1/2), that is, 1/180. In a practical example, the goodoperation of the system in the specific case where the extraction gridhad a 4-cm diameter and where the plasma chamber was a cylinder with a20-cm height and a 25-cm diameter has been confirmed. In this case,S₂=12.5 cm² and S₁=2550 cm², ratio S₂/S₁ thus is 1/204, which fulfillsthe required condition. The electron current that could be extractedfrom the plasma was practically 0.5 ampere under an extraction voltageV₂-V₁ of 60 V.

Of course, the present invention is likely to have various alterations,modifications, and improvements which will readily occur to thoseskilled in the art. In particular, the plasma may be formed from gasesother than argon, for example, lighter gases such as hydrogen or helium.

Further, it will be within the abilities of those skilled in the art tocomplete the electron source illustrated in FIG. 1 with the appropriatedevices necessary to its operation, described for example in Frenchpatent application No 99/10291. Similarly, processing enclosure 3 may becompleted in any appropriate fashion. A post-acceleration grid orelectrode may for example be provided between extraction grid 2 of theelectron source and processing enclosure 3.

1. A source providing an electron beam of settable power, comprising aplasma chamber (P) formed of an enclosure (1) having an internal surfacearea of a first value (S₁) and of an extraction grid (2) having asurface area of a second value (S₂), the grid potential being differentfrom that of the enclosure and being settable, characterized in that theplasma is excited and confined in multipolar or multidipolar magneticstructures, the ratio of the second value (S₂) to the first value (S₁)being close to the following upper limit:${D = {\frac{1}{\beta}\sqrt{\frac{2\quad\pi\quad m_{e}}{m_{i}}}{\exp\left( {{- 1}/2} \right)}}},$where: β is the electron ratio of plasma P, m_(e) is the mass of theelectron, and m_(i) is the mass of the positively-charged ions, wherebythe electron current density of the extracted beam is substantiallysteady when the grid-enclosure potential difference varies.
 2. Thesource of claim 1, characterized in that the plasma is excited andconfined by microwaves at the distributed electron cyclotron resonance.3. The source of claim 1, characterized in that the ratio between thesecond value (S₂) and the first value (S₁) is selected to be betweenD/10 and D.
 4. The source of claim 1, characterized in that the ratiobetween the second value (S₂) and the first value (S₁) is selected to bebetween D/2 and D.
 5. The source of claim 1, characterized in that thegrid is divided into a plurality of elementary grid portions (S₂₁, S₂₂,. . . S_(2n)).
 6. The source of claim 5, characterized in that variousgrid portions are gathered and set to distinct potentials.
 7. The sourceof claim 1, characterized in that it is followed by a post-accelerationenclosure.
 8. The source of claim 1 or 5, characterized in that the gateis formed of parallel tubes run through by a cooling fluid.